1. The immediate contents of experience which constitute the subject-matter of psychology, are in all cases processes of a composite character. Sense perceptions of external objects, memories of such sense perceptions, feelings, emotions, and volitional acts, are not only continually united in the most various ways, but each of these processes is itself a more or less composite whole. The idea of an external body, for example, is made up of partial ideas of its parts. A tone may be ever so simple, but we localize it in some direction, thus bringing it into connection with the idea of external space which is highly composite. Every feeling is referred to some sensation that aroused the feeling, and every volition is referred to an object willed. In dealing with a complex fact of this kind, scientific investigation has three problems to be solved in succession. The first is the analysis of composite processes; the second is the demonstration of the combinations into which the elements discovered by analysis enter; the third is the investigation of the laws that are operative in the formation of such combinations.

2. The second, or synthetic, problem is made up of several partial problems. In the first place, the psychical elements unite to form composite psychical compounds which are separate and relatively independent of one another in the continual flow of psychical processes. One group of examples of such compounds is to be found in ideas, whether referred directly to external impressions or objects, or interpreted by us as memories of impressions and objects perceived before. Other examples are composite feelings, emotions, or volitions. Then again, these psychical compounds stand in the most various interconnections with one another. Thus, ideas unite to form larger simultaneous ideational complexes or regular successions, while affective and volitional processes form combinations with one another and with ideational processes. In this way we have the interconnection of psychical compounds as a class of synthetical processes of the second order, consisting of a union between the simpler combinations of the first or lower order, which result from the combinations of elements into psychical compounds. The separate psychical interconnection of the second order unite in turn to form still more comprehensive combinations, which also show a certain regularity in the arrangement of their components. In this way, combinations of a third order arise, which we designate by the general name psychical developments. These may be divided into developments of different scope. Developments of a more limited sort are such as relate to a single phase of mental activity, for example the development of the intellectual functions, of the will, or of the feelings, or of merely one special branch of these functions, such as the aesthetic or moral feelings. From a number of such partial series arises the total development of a psychical personality. Finally, since animals, and in a still higher degree human individuals, are in continual interrelation with their fellow beings, there arise above these individual forms, general psychical developments. These various branches of the study of psychical development are in part the psychological foundations of other sciences, such as the theory of knowledge, pedagogy, aesthetics, and ethics, and are, accordingly, treated more appropriately in connection with those subjects. In part they have become special psychological sciences, such as child-psychology, animal-psychology and social psychology. We shall, therefore, in this treatise discuss only those results from the last mentioned departments which are of the most importance to general psychology.

3. The solution of the last and most general psychological problem, namely, the problem of discovering the principles and general laws of psychical phenomena, depends upon the investigation of all the combinations of different orders, that is, the combinations of elements into compounds, of compounds into interconnections, and of interconnections into developments. And as this investigation is the only means by which we can learn the actual composition of psychical processes, so also the only means of discovering the attributes of psychical causality, which finds expression in these processes, is in the investigation of the laws followed by the contents of experience and their components in their various combinations.

We have, accordingly, to consider in the following chapters: